1. Field of the Invention
The present invention relates to a plasma X-ray source for generating high power and highly stable soft X-rays to be used in an exposure apparatus for replicating a fine pattern to be used in the fabrication of semiconductor integrated circuits and others.
2. Description of the Prior Art
With a trend toward higher density integration in integrated circuits, a fine and highly accurate replicating technique has become necessary. Such a technique includes an X-ray lithography method. The X-ray source of an X-ray lithography apparatus has heretofore employed an electron impact system which generates X-rays by bombarding a solid such as aluminum, silicon, or palladium, with electron beams. However, this has involved a problem of such a low productivity that no high output of X-rays can be obtained at a low X-ray generation efficiency in the order of 10.sup.-4. An X-ray source which solves this problem is a plasma X-ray source which can be expected to generate X-rays with a higher output at a higher X-ray generation efficiency as compared with the electron impact X-ray source. Plasma X-ray sources include those of capillary discharge, plasma focus, and gas injection discharge types. In the capillary discharge type plasma X-ray source, vaporized polyethylene forms a high density plasma which generates only X-rays with long wavelengths. Thus, it is not suitable for X-ray lithography. On the other hand, the plasma focus discharge type X-ray source involves a problem of poor X-ray output stability due to contamination of an insulator surface which generates discharge initially. In contrast to these plasma X-ray sources, the gas injection discharge type X-ray source provides wavelengths suitable for X-ray lithography, and has a good X-ray generation stability.
Stallings et al. disclose X-ray emission from an imploding argon plasma ("Imploding argon plasma experiments", Appl. Phys. Lett. 35 (7), 1979, pp. 524-526). Okada et al. disclosed an X-ray source in which X-rays are extracted in the axial direction of a pinched plasma, and an X-ray lithography method which utilizes such an X-ray source ("X-ray source and X-ray lithography method", U.S. Ser. No. 699,402 filed Feb. 7, 1985, European Patent Application No. 85101451.4 filed Feb. 11, 1985). Pearlman et al. and Bailey et al. disclosed X-ray lithography in which X-rays are extracted in the radial direction of a pinched plasma ("X-ray lithography using a pulsed plasma source", J. Vac. Sci. Technol., 19 (4). Nov./Dec. 1981, pp. 1190-1193) and ("Evaluation of the gas puff Z pinch as an X-ray lithography and microscopy source", Appl. Phys. Lett. 40 (1), 1 Jan. 1982, pp. 33-35), respectively.
FIG. 1 shows an example of a conventional X-ray lithography apparatus in which X-rays are extracted in the axial direction of a plasma by using a gas injection discharge type plasma X-ray source, and which includes a vacuum casing 1, a vacuum chamber 2, a vacuum pump 3, and an upper electrode 4 and a flange 4B for supplying an electric current to the upper electrode 4. A gas passage 4C is provided in the upper electrode 4. The flange 4B includes a fast acting puff valve 5 composed of a gas plenum 5A and a piston 5B. The gas plenum 5A is connected to the upper gas passage 4C in the upper electrode 4 via a gas passage 5C. Illustration of a means for driving the piston 5B in the figure is omitted. A lower electrode 6 has a mesh or an opening 6A confronting the upper electrode 4 and openings 6C for gas evacuation. Reference numeral 6B is a flange for supplying an electric current to the lower electrode 6. FIG. 1 further includes a gas jet 7 ejected from the gas passage 4C, a pinched plasma 8, X-rays 9 generated, a particle beam 10, an X-ray extraction window 11, a mask 12, a wafer 13, a charged particle remover 14 comprising two permanent magnets, and an insulator 15 for insulating the flanges 4B and 6B, and the vacuum casing 1. The flange 6B is connected with a condenser 16 via a lead wire 18. The flange 4B is connected with one side of a discharge switch 17, the other side of which is connected with the condenser 16.
Gas injection discharge is performed as follows. The vacuum chamber 2 is evacuated with a vacuum pump 3 to about 10.sup.-5 to 10.sup.-6 Torr, while a discharge gas such as neon or krypton is introduced from a gas bomb 19 into the fast acting puff valve 5. After charging the condenser 16 with a charging power source 20, a power source 22 for the fast acting puff valve 5 is operated in response to a signal from a signal generator 21 to drive the fast acting puff valve 5. Thus, a gas jet is formed between the upper electrode 4 to which a high voltage is applied and the confronting lower electrode 6, while at the same time a signal from the signal generator 21 is inputted into a high voltage pulse generator 24 via a delay unit 23 which is so set to allow the time of discharge gas injection between the upper electrode 4 and the lower electrode 6 to agree with the time of discharge initiation. The discharge switch 17 is operated by a high voltage pulse to apply a high voltage across the upper electrode 4 and the lower electrode 6 which are illustrated by the insulator 15, to ionize the gas jet 7. Thus, a cylindrical plasma is formed. The plasma is converged by the interaction of the magnetic field formed by an electric current flowing along the direction of the central axis of the cylindrical plasma (hereinafter referred to as the "direction of the plasma axis") with ions and electrons in the plasma. Thus, the plasma is compressed to form a high temperature and high density plasma 8. X-rays 9 are generated by the interaction of ions and electrons in the high temperature and high density plasma 8. Besides X-rays, a particle beam 10 composed of charged particles such as ions and electrons, and a high temperature gas is emitted. A large amount of a high energy particle beam 10 is radiated in the direction of the central axis of the electrode.
In the case of exposure in the direction of the plasma axis which is suitable for fine pattern replication due to a small diameter of the source of X-rays, the damage of the X-ray extraction window 11 due to particles and light with a high energy which come flying in the direction of the plasma axis grow serious. In order to avoid the damage due to the charged particles during exposure in the direction of the plasma axis, use has heretofore been made of a method in which the charged particle remover 14 having mutually confronting magnets is inserted between the pinched plasma 8 and the X-ray extraction window 11 as shown in FIG. 1 to remove the charge particles by the magnetic field of the charged particle remover. In this method, however, almost all of the charged particles reflected from the upper electrode having a gas injection passage are radiated toward the X-ray extraction window. And the amount of the charged particles so notably increase in th direction toward the X-ray extraction window that the charged particles cannot be perfectly removed by only the charged particle remover. Use has also been made of an improved method in which a plasma reflection plate is disposed as a plasma remover having an X-ray passage hole in the center thereof in the magnetic field of the charged particle remover 14 to remove the charged particles, the high temperature gas, etc. due to the effect of not only deflection of the charged particles by the magnetic field but also reflection of the plasma by the plasma reflection plate. However, their removal cannot be perfect yet. In view of this, use has been made of an X-ray extraction window made of a thick beryllium film for preventing the X-ray extraction window 11 from being damaged. However, this increases X-ray attenuation to provide such a poor X-ray extraction efficiency as to prolong the period of time required for X-ray lithography, thus leading to a problem of reduction in throughput.
In order to avoid non-uniform exposure, 10 to 20 shots of discharge are repeated in one field exposure. Repeated discharge involves repetition of evacuation, gas injection, and discharge. In conventional apparatus, however, an increase in the speed of discharge repetition entails such insufficient gas evacuation around the discharge electrodes 4 and 6 that abnormal discharge in portions other than the vicinity of the electrodes 4 and 6 is liable to occur, leading to reduction in the X-ray output. Therefore, a repetition frequency of about 1 Hz must be adopted for securing a stable X-ray output. Thus, no improved throughput can be expected.
When a plasma is pinched and collapsed, a gas which collides with the upper electrode and is reflected from the upper electrode is radiated toward the X-ray extraction window. This results in such insufficient exhaustion of the gas present in the direction of the X-ray extraction window during high frequency repetition of discharge that X-rays advancing toward the X-ray extraction window are attenuated by the gas. As shown in FIG. 2, the output of X-rays with a wavelength of 12 .ANG. are decreased about 1/2 when a neon gas of 70 Torr is present in a thickness of about 1 to 2 cm around the center of the lower electrode.
Since a high temperature and high density plasma is radiated in the direction of the plasma axis, the central portion of the upper electrode is largely lost and the molten electrode substance is scattered to contaminate the X-ray extraction window.
FIG. 3 shows another conventional X-ray lithography apparatus. In order to avoid the influence of a particle beam comprising charged particles, a high temperature gas, etc., this apparatus utilizes a phenomena that the radiation dose of the particle beam 10 in the radial direction of the plasma is 1/100 to 1/1000 of that in the direction of the plasma axis. More specifically, the X-ray extraction window 11, the mask 12, a wafer, etc. are disposed in the radial direction of the pinched plasma 8 to extract X-rays through a hole 6D provided in the electrode 6 and the X-ray extraction window 11 for effecting exposure. Illustration of a discharge gas feed system and an electric system for discharging is omitted in FIG. 3. FIG. 4 is a form of an X-ray source photographed from the radial direction of the X-ray source in which direction an X-ray mask is disposed, according to X-ray pinhole photography. In the case of such exposure in the radial direction, when proximity lithography is performed at a mask-wafer gap of 10 to 20 .mu.m, the penumbral blur .delta.=ds/D, which is determined by the length d of a pinched plasma, namely a source of X-rays, the distance D to the source of X-rays, and the distance s between the mask and the wafer as shown in FIG. 5 so increase that fine pattern replication cannot be made.